✔ 最佳答案
圖片參考:
http://imgcld.yimg.com/8/n/HA00132885/o/701110020106713873470710.jpg
角SOR = 70 ( alt. angle, PS//OR )
角SOR = 2 x 角 SPR (angle at center = 2 x angle at circumference)
a = 角SPR = 角SOR / 2 = 35
角PRO = 角SPR = 35 ( alt. angle, PS//OR )
角OPR = 角PRO = 35 ( OP = OR, 三角形POR 是等腰三角形)
角POR = 180 - 35 - 35 = 110
角POR = 2 x 角PQR (angle at center = 2 x angle at circumference)
b = 角PQR = 角POR / 2 = 55